Sliding caliper disc brake assemblies have been in use in automotive applications for many years. For example, in a passenger vehicle at least one brake pad is supported by a metal caliper guide pin which typically slides within an elastomeric bushing, the latter being supported by a member which is stationary with respect to the body of the vehicle. When actuated, the guide pin urges the brake pad against a disc which is generally mounted on the vehicle's wheel and the resulting frictional forces reduce the speed of the moving vehicle or bring it to a complete stop. In order for the brake assembly to operate efficiently, the guide pin must slide freely within its surrounding bushing and therefore the contact area between guide pin and bushing must remain well lubricated. In this regard, the bushing serves at least three functions: (1) it contains any lubricant employed, (2) it eliminates metal-to-metal contact and the wear and noise associated therewith and (3) it excludes dirt, water and other corrosive elements which detract from smooth operation.
However, even with the best current disc brake designs which employ specially formulated lubricants, there is still a certain hysteresis associated with the sliding motion of the guide pin. This phenomenon is called "stiction," which is defined for the purposes of the present invention as the force required to overcome the static friction between the guide pin and the bushing at any given position within the pin's range of travel. Stiction typically increases with time as the brake assembly sits at rest for prolonged periods and this inactivity further reduces brake performance. Moreover, the brake assembly is often exposed to harsh environments during normal use (e.g., water, road salt, dirt, oil) and such exposure can greatly accelerate deterioration of the lubricant and promote corrosion of the guide pin. This, in turn, can result in increased stiction, reduced brake reliability and, eventually, brake failure. further, the lubricant must be compatible with the elastomeric bushing and should not excessively swell or otherwise adversely affect the bushing.
Various compositions which can be used as a grease or lubricant in the above described application are available commercially, but all of these systems are found lacking in one or more of the aforementioned desired characteristics. Thus, for example, petroleum-based greases tend to swell the elastomeric bushing and cause premature system failure. Greases based on poly alkylene glycol (PAG) or silica-filled silicone generally exhibit high stiction in this application. Greases based on perfluoropolyether (PFPE) are often too expensive for automotive application. There is therefore a need for improved compositions which can be used to lubricate the guide pin/bushing combination of the above described disc brake assemblies.